Video disc systems are known in which a video program is recorded or stored for playback through a television receiver in a process which is generally similar to the playback of audio records. While such systems may operate on a variety of different principles, the optical system is of particular interest with respect to the present invention which will be described in that environment.
One form of video disc for the optical system has a succession of pits alternating with lands usually in a multiturn spiral storage track with the pits and lands of such varying dimension along the length of the track as to collectively constitute a spatial representation of a carrier signal frequency modulated with program information. The program generally includes luminance, chroma, synchronizing and audio information. Scanning of the storage track with a reading beam of energy effects modulation of the beam so that its impingement upon a photoreceptor develops a frequency modulated carrier wave signal conveying the information stored in the disc. It is of course known that the reading or playback system may operate in the transmissive mode in which case the record is transmissive to the reading beam and the pits serve as a diffraction pattern to modulate the beam as it passes through the disc. Alternatively, the playback may function in the reflective mode which is generally similar except that in this case the reading beam, instead of passing through the disc, is reflected therefrom to a suitably positioned photoreceptor.
Experience has shown that the pits and lands frequently suffer a length distortion so that dimensionally they do not have the 50% duty factor required accurately to match or represent the modulated carrier signal under the control of which these elements are formed in the process of storing information in the disc. This may be likened to a modulation in duty cycle or an unintended and undesired incremental variation in the length of such elements measured along the storage track. Most generally, the pit length suffers an increase but whether the duty factor is increased or decreased its effect is to introduce a band of spurious components into the base band of the output signal derived from reading the disc. Where the nominal frequency of the frequency modulated carrier signal utilized in storing information in the disc is at least twice the highest video frequency component desired to be reproduced, the spurious signal content attributable to undesired modulation of duty factor can be eliminated by filtering. On the other hand, if the carrier frequency is less than twice the highest video component, the band of spurious signal components attributable to duty factor modulation overlaps and occasions interpenetration with the lower sideband components of the carrier signal derived from reading the disc. In such a case, the spurious components can no longer be avoided by filtering and they tend to introduce imperfections in image reproduction.
One might suppose that the air would always choose a carrier frequency to facilitate obviating imperfections in image reproduction caused by duty factor modulation or error except that there are desired advantages in operating the system with low values of carrier frequency. It is found, for example, that with lower carrier frequencies there is a less stringent resolution requirement because of the increased spatial wave length and also the depth of focus may be increased since that parameter is related to spot size. And finally, the optical system with its tracking arrangement and optical components may be less expensive and more practical when designed for lower carrier frequencies.